nonnest2: Tests of Non-Nested Models

Testing non-nested models via theory supplied by Vuong (1989) <doi:10.2307/1912557>.
Includes tests of model distinguishability and of model fit that can be applied
to both nested and non-nested models. Also includes functionality to obtain
confidence intervals associated with AIC and BIC. This material is partially based on
work supported by the National Science Foundation under Grant Number SES-1061334.